1. Field of the Invention
The present invention relates to a power source device and a charge controlling method to be used in the power source device, and more particularly to the power source device in which electric power generated by a solar cell is temporarily stored in an electric double-layer capacitor and which has a secondary cell to be charged based upon the electric power stored in the electric double-layer capacitor and which is suitably used, for example, even in a place where power infrastructure is not provided, and to the charge controlling method to be used in the above power source device.
The present application claims priority of Japanese Patent Application No. 2004-145705 filed on May 14, 2004, which is hereby incorporated by reference.
2. Description of the Related Art
A power source device having a secondary cell to be charged by a solar cell that converts solar light energy into electricity is being used as a power source for electronic devices or a like in a place where power infrastructure is not provided. However, the conventional power source device presents a problem in that power to be generated by the solar cell is not stable due to changes in the surrounding environments such as variations in intensity of sunlight (solar irradiation) and in ambient temperatures. To solve these problems, a power source device to improve such problems as described above is proposed.
One of such conventional power source devices is disclosed, for example, in Japanese Patent Application Laid-open No. 2002-238182 (See Page 1, FIG. 1 within the same Patent Document) in which a solar cell and an electric double-layer capacitor are connected in parallel and a diode array to produce a reference voltage used to determine an operating point of the solar cell is placed very near to the solar cell and, as a result, the diode array and the solar cell are thermally coupled, which makes the reference voltage value coincide with a temperature characteristic of the solar cell. As a result, the operating point of the solar cell becomes a most suitable operating point at which maximum efficiency is obtained and, while the operating point is sequentially corrected so as to respond to a change in the most suitable operating point caused by a change in temperature of the solar cell, a battery is charged through a switching device and a constant current DC (Direct Current)-DC converter.
However, the above conventional power source device has the following problems. That is, for example, in the above conventional power source device, if the solar cell is mounted in a place being far from the power source device, since a circuit to produce the reference voltage is separated from the power source device, an electric wiring between the circuit to produce the reference voltage and the power source device is made long and as a result, appearance of electric noises and a voltage drop caused by wiring resistance occur, which cause accuracy of the reference voltage to be degraded and the operating point of the solar cell not to be at the optimum value. Another problem is that, if the battery, having a single-piece construction such that the solar cell and the power source device are integrated together, is placed outdoors, a temperature of the battery exceeds its upper limit in a use temperature range, in a high temperature environment, for example, in a state of mid-summer with no breeze and, as a result, there is a risk that the battery produces heat or breakdown. Moreover, in the case of constant-current charging in a low temperature environment, there is a possibility that charging efficiency decreases due to an increase in internal impedance of the battery itself. Furthermore, even when only the battery is put indoors to moderate temperature environments, there is a possibility that a decrease in efficiency in the constant-current charging occurs due to an increase in resistance of wirings to connect the battery to the power source device.
There is still another problem in that, when electric power generated by the solar cell is low in such a state in which the intensity of sunlight is low in the morning or the evening, on a cloudy day, or a like, a long time is required to increase a charging voltage up to the most suitable operating point due to a leak current in the electric double-layer capacitor or to power consumed in a static state in internal circuits of the power source device and a period required to put the switching device in an ON state is made long, which causes the charging efficiency to be degraded.